The present invention relates generally to medical diagnostic tests, and more specifically to metabolic breath tests utilizing isotope ratios.
Certain diagnostic medical tests rely on administering an isotopically labelled compound to a patient and then monitoring the patient's breath for metabolic products of the labelled compound. Traditional metabolic breath tests rely on administering a .sup.14 C-labelled substrate to a patient and then measuring the concentration of .sup.14 CO.sub.2 exhaled on the breath. Since .sup.14 C is radioactive, it is easily detected with inexpensive radiation monitoring equipment. Unfortunately, this radioactivity is also a problem because of its health risk to the patient. One approach has been to replace the .sup.14 C isotope with a non-radioactive one, such as .sup.13 C. Although this eliminates the risks associated with exposure to .sup.14 C, it creates a new problem; how to detect .sup.13 CO.sub.2 on the breath. This problem is further compounded by the fact that the relative natural abundance of .sup.13 C is approximately 1%, and considerable variability in this value is known to exist. Although a number of traditional approaches exist for monitoring .sup.13 C, including isotope-ratio mass spectrometry and nuclear magnetic resonance spectrometry, the associated instrumentation is exceedingly expensive and therefore limits the widespread use of .sup.13 C labelled compounds in diagnostic tests.
Lee and Majkowski (U.S. Pat. No. 4,684,805) consider the use of cryogenically cooled tunable infrared lead-salt laser diodes for this as well as other medical tests by monitoring certain molecular species on human breath. Since the strongest absorption lines of interest were in the 4-.mu.m to 5-.mu.m range, lead-salt laser diodes, which emit in the 3-.mu.m to 30-.mu.m region were a natural choice. However, lead-salt laser diodes and their associated detectors operate at liquid nitrogen temperatures. Furthermore, their output is generally multimode and is typically less than a milliwatt.